To provide cellular wireless communication service, a wireless service provider typically employs a radio access network (RAN) that functions to connect one or more wireless communication devices (WCDs) with one or more transport networks, such as the public switched telephone network (PSTN) and the Internet. In a typical RAN, an area is divided geographically into a number of coverage areas, such as cells and sectors, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS). Within each coverage area, the BTS's RF radiation pattern may provide one or more frequency channels over which WCDs may communicate with the RAN. In turn, the RAN may provide connectivity with other served WCDs and/or with entities on a transport network.
The RAN's frequency channels may each carry communications between the RAN and the WCDs according to an agreed air-interface protocol, examples of which include CDMA (e.g., 1×RTT or 1×EV-DO), LTE, WiMAX (e.g., IEEE 802.16), GSM, WIFI (e.g., IEEE 802.11), Bluetooth, and other protocols now known or later developed. Generally, the agreed air-interface protocol may divide a frequency channel into a “forward link” for carrying communications from the RAN to the WCDs and a “reverse link” for carrying communications from the WCDs to the RAN. And the agreed air-interface protocol may further divide the frequency channel's forward and reverse link into a plurality of sub-channels (also themselves referred to as channels), such as shared control channels for carrying control data (e.g., pilot signal, registration messages, call-setup messages, system parameter messages, etc.) and dedicated traffic channels for carrying bearer-traffic data.
In a typical arrangement, a RAN may provide, in each coverage area, one or more reverse-link control channels—known as an access channels—that are generally available for use by WCDs to send “access probes” (e.g., registration messages, call-setup messages, etc.) to the RAN. Correspondingly, the RAN may periodically broadcast, over at least one forward-link control channel of the coverage area, messages that identify the coverage area's available access channel(s). For example, the RAN may periodically broadcast “channel list messages” (CLMs) that identify each available frequency channel in the given coverage area. In addition, the RAN may also periodically broadcast “access parameters messages” (APMs) that identify each available access channel on each available frequency channel in the coverage area. The RAN may broadcast other such messages as well.
Within such an arrangement, when a WCD wishes to register with the RAN in a coverage area, the WCD may first identify the one or more access channels that are generally available for use by WCDs to access the RAN in the coverage area (e.g., based on the CLMs and/or APMs). In turn, the WCD may then carry out an access-channel selection process to select an available access channel in the coverage area for use to transmit a registration attempt (which may take the form of one or more messages indicating that the WCD wishes to receive service from the RAN in the coverage area). Depending on the coverage area's configuration and/or the air-interface protocol employed, this selection process may take various forms.
For instance, if the WCD identifies only a single access channel in the given coverage area, the WCD may simply select that single access channel to use for transmission of a registration attempt. On the other hand, if the WCD identifies multiple access channels in the coverage area the WCD may execute one or more “hashing algorithms” keyed on one of the WCD's identifiers (e.g., a serial number, directory number, etc.) to select which of the coverage area's access channels to use for transmission of its registration attempt. The WCD may select an access channel in other manners as well.
After making its selection, the WCD may then send its registration attempt to the RAN over the selected access channel in the coverage area. Thereafter, the WCD may operate in an “idle” mode in the coverage area during which the WCD maintains its selection of (and thus continue to operate on) the selected access channel for use to transmit access probes to the RAN until the WCD moves to a new coverage area, detects a change in the coverage area's available access channel(s), etc. Further, if any of the WCD's access probes ultimately result in a new communication session with the RAN in the coverage area, the WCD may transition into an “active” mode and begin engaging in traffic-channel communication with the RAN.